1. Field of the invention:
This invention relates to a semiconductor device such as a semiconductor laser device, a transistor, etc., which contains semiconductor multiple thin films formed by molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc.
2. Description of the prior art:
Recently, a single crystal growth technique for the formation of thin films such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc., has been developed which enables the formation of thin film growth layers having a thickness of as thin as approximately 10 .ANG.. The development of such a technique allowed the production of the thin film layers to be applied to laser devices. However, these significantly thin film layers have not yet been produced by liquid phase epitaxy (LPE).
An example of these laser devices is a superlatticed device having a periodic multiple-layered structure of plural semiconductor thin films. Typical superlatticed devices are composed of alternate layers consisting of two kinds of semiconductor thin films (the thickness thereof being approximately 100 .ANG. or less) which are typically made of Ga.sub.1-x Al.sub.x As (0x.ltoreq.1). Since the lattice constants of two different Ga.sub.1-x Al.sub.x As materials are well matched, lattice defects based on internal stresses are rare.
The main feature of superlattices is that multiple-layered thin films are formed with a periodicity which is longer than the lattice constant (several .ANG.), resulting in repeat of the band structure in the reciprocal lattice space thereof. Such a deformity of the band structure allows the regulation of electrical characteristics of the superlattices. For instance, a superlattice which is composed of alternate layers consisting of two GaAs layers and two AlAs layers provides a multiple-layered structure having a periodicity which is two times the lattice constant, resulting in a band structure which is folded to the two halves of the Brillouin band in the reciprocal lattice space in the lamination direction. As mentioned above, the main feature of superlattices is that the band structure can be changed without changing the kinds of crystal elements. However, so long as Ga.sub.1-x Al.sub.x As materials are used for the formation of superlattices, the change of the band structure is attained only by either a change of the AlAs mole fraction x or a change of the layer thickness of the two kinds of Ga.sub.1-x Al.sub.x As layers, so that a change of the band structure can be only attained in a limited range.
On the other hand, in the case where even such bulk crystals such as a SiC crystal are of a polytype (i.e., 2H type, 3C type and 6H type), even if one of the various type crystals had the same elements and composition ratio as another, various crystals having different crystal structure and different band structure can be obtained when the elements of each of the crystals are arranged with a different periodicity. That is, with regard to the lamination of SiC layer units in the direction of the C-axis, three kinds of lattice positions (hereinafter referred to as A, B and C) exist; for example, the 2H type crystal provides a superlatticed structure of ABAB . . . , the 3C type crystal provides a superlatticed structure of ABCABC . . . and the 6H type crystal provides a superlatticed structure of ABCACBABCACB . . . . Moreover, the energy gaps of the 2H type, the 3C type and the 6H type are 3.33 eV, 2.39 eV and 3.02 eV, respectively, which are quite different from each other. Artificial polytype superlattices can be produced by the use of three or more kinds of crystal elements. For instance, a superlattice which results from the periodical lamination of InSb layers (the thickness of each layer being 50 .ANG.), GaSb layers (the thickness of each layer being 50 .ANG.) and AlSb layers (the thickness of each layer being 50 .ANG.) is different in band structure from another superlattice which results from the periodical lamination of InSb layers (the thickness of each layer being 50 .ANG.), GaSb layers (the thickness of each layer being 50 .ANG.), InSb layers (the thickness of each layer being 50 .ANG.) and AlSb layers (the thickness of each layer being 50 .ANG.). However, these polytype superlattices must be composed of three or more kinds of crystal elements and cannot be produced using the combination of GaAs and AlAs, the lattice constants of which are well matched.
On the other hand, superlattices can be designed using the combination of semiconductor thin layers (e.g., GaAs.sub.1-x P.sub.x (x&lt;1) and GaP, In.sub.x Ga.sub.1-x As (x&lt;0) and GaAs, etc.), the lattice constants of which are not matched. When the thickness of each layer of these superlattices, which are so-called strained superlattices, is approximately 100 .ANG. or less, the internal stress due to the matchless lattice constants therebetween is too small to cause dislocation and accordingly the resulting crystals have almost no lattice defects therein. Thus, the use of such strained superlattices enlarges the degree of freedom in the choice of materials. Since energy gaps of superlattices are generally greater than those of the semiconductor thin layers constituting the superlattices, the use of a superlattice for the active region of a semiconductor laser device enables the shortening of the oscillation wavelength of laser light which is produced by the device. However, when strained superlattices are employed for the active region, the rapid formation of lattice defects and the multiplication thereof occur, making it difficult to put such strained superlattices to practical use.